1. Field of the Invention
The invention relates to the technical field of inkjet printing and, more particularly, to an inkjet printing system and method capable of automatically calibrating non-uniform speed of a printhead carriage.
2. Description of Related Art
For current inkjet printing technology, the print quality and the throughput generally conflict and require a compromise. For example, a multi-pass printing is required for a photo-level print quality. In this case, the printhead reciprocates in a carriage scan direction many times to thereby gradually produce a complete image output.
An example is given by a desktop photo printer with model number 6578 of Hewlett-Packard cooperation. For an A4 photo printout by the HP 6578, the printhead requires the reciprocation for printing by 400 swatches. For each swatch, the printhead carriage is carried with the course of start, acceleration, uniform motion and retardation by the driver motor. Therefore, the printout speed is a bottleneck of the current inkjet printing technology.
In addition, for a typical inkjet printing, the ink ejection is carried out when the printhead enters in a uniform zone. As a consideration of the ejection speed of ink droplets in a horizontal direction, the printed image is distorted to result in printing defects or errors, and a poor print quality in case that the ink is ejected in accordance with a fixed clock and the printhead carriage starts to print as soon as the printhead carriage enters in an acceleration/retardation zone.
U.S. Pat. No. 5,997,130 granted to Bolash et. al. for an “asymmetrical acceleration ramp area and method for print cartridge carrier of ink jet printer” has disclosed an asymmetrical acceleration zone, which can reduce the working space without affecting the print quality. FIG. 1 is a graph schematically illustrating the speeds of a print cartridge carrier in the U.S. Pat. No. 5,997,130. As shown in FIG. 1, the speeds are not calibrated substantially since it only reduces the reserved space for the acceleration or retardation zone, wherein the reserved space for the retardation zone is reduced in this case. Accordingly, when a high quality output mode is selected in printing, it prints in a single direction, starts the printhead to print a swatch from one side that reserves the space for the acceleration zone, and keeps a uniform speed in this zone. As soon as the print zone is passed, a relative retardation is proceeded to stop the printhead. Because the bi-directional printing is not required, the problems associated with the acceleration and retardation zones are not considered in the backward travel of the printhead. However, for printing in a script mode, the required amount of ink droplets is considerably reduced, and the quality requirement is also reduced. In this case, the printing in bi-direction can be started before the printhead carriage reaches to the uniform zone, which affects the print quality but still meets with the requirement of the script mode.
U.S. Pat. No. 6,361,137 granted to Eaton et. al. for a “method and apparatus for compensating for variations in printhead-to-media spacing and printhead scanning velocity in an ink-jet hard copy apparatus” has disclosed a method of calibrating the speed on a printhead carriage, which uses a complicated hardware circuit to determine a carriage speed and a printing clock. The printing clock is a function of the motion speed on the carriage and the distance from the printhead to a paper. U.S. Pat. No. 6,361,137 does not print a test pattern so as not to read and analyze the result of a test pattern printing, and instead uses a droplet-flight-time measurement module to real-time analyze the instant speed in accordance with a received encoder signal with respect to the printhead travel, and to determine a measured flight time of ink droplets in a certain range for further determining a clock on the inkjet control signal. In addition, a system clock signal is interpolated between two encoder signals to thereby measure the speed presented on a time axis and the flight time of ink droplets. Thus, the inkjet clock is determined. However, this requires a complicated hardware to determine the carriage speed and the printing clock.
U.S. Pat. No. 5,448,269 granted to Beauchamp et. al. for a “multiple inkjet cartridge alignment for bidirectional printing by scanning a reference pattern” has disclosed a method of calibrating a placement error caused by the initial exit speed of ink droplets. U.S. Pat. No. 5,448,269 calibrates the placement error caused by a curve of the paper roller in a paper advance direction. Namely, the speed is calibrated at a bi-directional print mode. In U.S. Pat. No. 5,448,269, the method first prints a test pattern formed with vertical lines, then uses an optical sensor to read the test pattern, and finally analyzes the error to accordingly calibrate the ejection speed by adding additional delays to the inkjet clock for partial holes or orifices. The method essentially compensates for the error caused by the curve of the paper roller, not for the error caused by the non-uniform carriage speed. Hence, an improvement to the typical inkjet printing system is desired.